The ability to 'cook' grenades by releasing the trigger, so you can burn down the fuse time before throwing.

If you're holding a grenade and you pull the pin, I think by default it should immediately start to lower the fuse instead of when you throw the grenade. Instead, if you want to keep the grenade's trigger held so it doesn't start to go off you have to have the trigger on your controller held down when you pull the pin.

If you accidentally start the fuse by forgetting to hold the trigger, simply reinserting the pin resets the fuse so there aren't any awkward moments of pulling the pin and the grenade immediately detonating in your hand from when you cooked it earlier. Of course, if you take too long reinserting the pin the grenade will explode in your hand and kill you.